JP4884081B2 - Distributed feedback laser diode - Google Patents

Distributed feedback laser diode Download PDF

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JP4884081B2
JP4884081B2 JP2006149905A JP2006149905A JP4884081B2 JP 4884081 B2 JP4884081 B2 JP 4884081B2 JP 2006149905 A JP2006149905 A JP 2006149905A JP 2006149905 A JP2006149905 A JP 2006149905A JP 4884081 B2 JP4884081 B2 JP 4884081B2
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phase shift
dfb
shift amount
current
diffraction grating
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JP2007324196A (en
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昌太郎 北村
康隆 阪田
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Renesas Electronics Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/028—Coatings ; Treatment of the laser facets, e.g. etching, passivation layers or reflecting layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H01S5/124—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers incorporating phase shifts

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  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
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  • Semiconductor Lasers (AREA)

Description

本発明は、分布帰還型半導体レーザに関する。   The present invention relates to a distributed feedback semiconductor laser.

光通信技術の分野においては、使用される単一縦モード発振する半導体レーザとして、分布帰還型半導体レーザ（ＤＦＢ−ＬＤ（Distributed FeedBack Laser Diode））が使用されている。例えば、ＤＦＢ−ＬＤは、変調速度１．０Ｇｂｐｓ以上、距離２０ｋｍ以上の光信号伝送、特に２．５Ｇｂｐｓ以上、５０ｋｍ以上の光信号伝送、において使用されている。   In the field of optical communication technology, a distributed feedback semiconductor laser (DFB-LD (Distributed FeedBack Laser Diode)) is used as a semiconductor laser that oscillates in a single longitudinal mode. For example, the DFB-LD is used in optical signal transmission at a modulation rate of 1.0 Gbps or more and a distance of 20 km or more, particularly, for optical signal transmission of 2.5 Gbps or more and 50 km or more.

ＤＦＢ−ＬＤにおいては、単一モード発振を実現するために、一般的に、位相シフト量がΛ／４の位相シフト部を回折格子に設けている（例えば、非特許文献１参照）。なお、本発明においては、位相シフト量は、光の発振波長「λ」ではなく、回折格子間隔の２倍を示す「Λ」で表記してある。   In the DFB-LD, in order to realize single mode oscillation, a phase shift unit having a phase shift amount of Λ / 4 is generally provided in the diffraction grating (see, for example, Non-Patent Document 1). In the present invention, the phase shift amount is expressed not by the light oscillation wavelength “λ” but by “Λ” indicating twice the diffraction grating interval.

この位相シフト量を調節したＤＦＢ−ＬＤとして、例えば特許文献１〜特許文献３に記載されたＤＦＢ−ＬＤが知られている。特許文献１においては、安定な単一軸モードでの動作を目的として、回折格子の位相シフト量をΛ／１６〜３Λ／８に設定したＤＦＢ−ＬＤを開示している（特許文献１参照）。特許文献２においては、変調時の波長チャープを抑制することを目的として、位相シフト量を（Λ／２−Λ／ｎ、ｎ＝４〜１６）に設定したＤＦＢ−ＬＤを開示している。また、特許文献３においては、安定な単一モードで発光効率を上げることを目的として、例えば位相シフト量Λ／８の位相シフト部を２つ形成したＤＦＢ−ＬＤを開示している。   As DFB-LDs in which the phase shift amount is adjusted, for example, DFB-LDs described in Patent Documents 1 to 3 are known. Patent Document 1 discloses a DFB-LD in which the phase shift amount of a diffraction grating is set to Λ / 16 to 3Λ / 8 for the purpose of operation in a stable single-axis mode (see Patent Document 1). Patent Document 2 discloses a DFB-LD in which a phase shift amount is set to (Λ / 2−Λ / n, n = 4 to 16) for the purpose of suppressing wavelength chirp at the time of modulation. Patent Document 3 discloses a DFB-LD in which, for example, two phase shift portions having a phase shift amount Λ / 8 are formed in order to increase the light emission efficiency in a stable single mode.

特開昭６３−３２９８８号公報JP 63-32988 A 特開２００３−２０４１１４号公報JP 2003-204114 A 特開２００３−１５２２７２号公報JP 2003-152272 A 稲葉文男監修、「レーザ工学入門」、社団法人電位情報通信学会編、平成９年発行Supervised by Fumio Inaba, "Introduction to Laser Engineering", edited by the Institute of Electrical and Potential Information Communication, 1997

ＤＦＢ−ＬＤにおいては、レーザ発振可能な波長が２つ（主モードとサブモード）ある。そこで、回折格子の位相シフトによって、一方の波長のみが選択的に発振される。このとき、２．５Ｇｂｐｓ以上の高帯域通信に利用されるＤＦＢ−ＬＤにおいては、異なる波長の群遅延による伝送不良を回避するために、主モードとサブモードとの強度比（サイドモード抑圧比（ＳＭＳＲ；Side Mode Suppression Ratio））が３０ｄＢ以上のものが要求されている。   In the DFB-LD, there are two wavelengths (a main mode and a sub mode) that allow laser oscillation. Therefore, only one wavelength is selectively oscillated by the phase shift of the diffraction grating. At this time, in the DFB-LD used for high-band communication of 2.5 Gbps or more, in order to avoid transmission failure due to group delay of different wavelengths, the intensity ratio (side mode suppression ratio (side mode) ( SMSR (Side Mode Suppression Ratio)) is required to be 30 dB or more.

しかしながら、回折格子に、位相シフト量がΛ／４の位相シフト部を設けたＤＦＢ−ＬＤにおいては、注入電流の増加に伴い、例えば、図９（ｂ）に示すように、発振モードである主モードがストップバンドの長波長側に移動し、短波長側のストップバンド端ピークが大きくなることがある。このような状態においては、図１０に示すように、主モードの単一発振が形成されない。   However, in a DFB-LD in which a diffraction grating is provided with a phase shift unit having a phase shift amount of Λ / 4, as the injection current increases, for example, as shown in FIG. The mode may move to the long wavelength side of the stop band, and the stop band end peak on the short wavelength side may increase. In such a state, as shown in FIG. 10, single oscillation of the main mode is not formed.

Λ／４シフトＤＦＢ−ＬＤにおいて、電流増加により単一発振が得られない理由は、以下のように考えられる。レーザ発振駆動時に、位相シフト部において、導波路の光密度が局所的に増大し、他の導波路部分よりもキャリアの消費が多くなる。これにより、位相シフト部のキャリア密度が局所的に低くなり（スペーシャル（空間的）ホールバーニング）、位相シフト部の導光波路屈折率が相対的に増大することになる。このため、位相シフト部の光学路長が、設定のΛ／４より実質的に大きくなってしまう。そこで、本発明においては、導波路屈折率の増大量を予め見込んで位相シフト量を設定する。すなわち、図１（ａ）に示すように、注入電流が閾値電流レベル（０．９Ｉｔｈ）の時には、主モードがストップバンド中央より短波長側にあるようにしておき、注入電流が駆動電流レベル（例えば３０ｍＡ）の時に、主モードがストップバンドの中心にくるように設定する。 The reason why a single oscillation cannot be obtained by increasing the current in the Λ / 4 shift DFB-LD is considered as follows. At the time of laser oscillation driving, the optical density of the waveguide locally increases in the phase shift unit, and the consumption of carriers is higher than that of other waveguide portions. As a result, the carrier density of the phase shift portion is locally lowered (spatial (spatial) hole burning), and the light guide waveguide refractive index of the phase shift portion is relatively increased. For this reason, the optical path length of the phase shift unit is substantially larger than the set Λ / 4. Therefore, in the present invention, the phase shift amount is set in advance by taking into account the increase amount of the waveguide refractive index. That is, as shown in FIG. 1A, when the injection current is at the threshold current level (0.9I th ), the main mode is set to be on the short wavelength side from the center of the stop band, and the injection current is at the drive current level. At (for example, 30 mA), the main mode is set to be at the center of the stop band.

本発明の第１視点によれば、位相シフト部を有する回折格子と、両端面に無反射膜を有する多重量子井戸型活性層と、を備える１．５５μｍ帯の分布帰還型半導体レーザであって、素子長は４５０μｍであり、結合係数は２．５又は３．５であり、位相シフト部は端部から１９５μｍの位置に形成され、位相シフト部の位相シフト量は、（８／４０）Λ〜（９／４０）Λ（Λは回折格子間隔の２倍）である分布帰還型半導体レーザを提供する。
According to a first aspect of the present invention , there is provided a 1.55 μm band distributed feedback semiconductor laser comprising a diffraction grating having a phase shift portion and a multiple quantum well active layer having antireflection films on both end faces. The element length is 450 μm, the coupling coefficient is 2.5 or 3.5, the phase shift unit is formed at a position of 195 μm from the end, and the phase shift amount of the phase shift unit is (8/40) Λ Provided is a distributed feedback semiconductor laser having (9/40) Λ (Λ is twice the grating interval).

なお、慣例的に「Λ／２−（本発明でいう位相シフト量）」を位相シフト量として呼ぶ文献も存在する。この位相シフト量は、発振するレーザ光に生ずる位相シフト量を意味している。この場合、「Λ」の代わりに「λ」を用いて記載される場合が多い。例えば、上記で説明したように、特許文献２においては、この定義が使用されている。本発明にこの定義を適用すると、位相シフト量（８／４０）Λ〜（９／４０）Λは、（１１／４０）λ〜（１２／４０）λという表記となる。なお、この「λ」は素子内における波長を意味している。すなわち、λ＝λ０／ｎ（λ０：真空中の波長λ０、ｎ：子導波路屈折率）と表記される。 In addition, there is a document that conventionally calls “Λ / 2− (phase shift amount in the present invention)” as a phase shift amount. This phase shift amount means the phase shift amount generated in the oscillating laser beam. In this case, “λ” is used instead of “Λ” in many cases. For example, as described above, this definition is used in Patent Document 2. When this definition is applied to the present invention, the phase shift amounts (8/40) Λ to (9/40) Λ are expressed as (11/40) λ to (12/40) λ. Note that “λ” means a wavelength in the element. That is, it is expressed as λ = λ 0 / n (λ 0 : wavelength λ 0 in vacuum, n: child waveguide refractive index).

本発明によれば、駆動電流が大きいとき（例えば通常レーザ発振動作時）に、主モードのピークをストップバンドの中央に移動させることができると共に、主モードとサブモードの抑圧比を大きくすることができる。これにより、単一モードで発振するＤＦＢ−ＬＤを得ることができる。本発明のＤＦＢ−ＬＤは、例えば、２．５Ｇｂｐｓ以上の高帯域通信に利用することができる。   According to the present invention, when the drive current is large (for example, during normal laser oscillation operation), the peak of the main mode can be moved to the center of the stop band, and the suppression ratio between the main mode and the sub mode can be increased. Can do. Thereby, a DFB-LD that oscillates in a single mode can be obtained. The DFB-LD of the present invention can be used for, for example, high bandwidth communication of 2.5 Gbps or higher.

本発明に係るＤＦＢ−ＬＤについて説明する。ＤＦＢ−ＬＤの構造を示す概略平面図を図２に示す。ＤＦＢ−ＬＤ１は、多重量子井戸型（ＭＱＷ；Multi-Quantum Well）の活性層４の下側に回折格子６を有し、さらに両端面に無反射膜（ＡＲコート、反射防止膜）７ａ、７ｂを有する。回折格子６は、位相シフト部６ａを有する。本発明における位相シフト部６ａの位相シフト量ｄは、（８／４０）Λ〜（９／４０）Λである。ここで、Λは、回折格子６の山（又は谷）の頂点間の間隔（回折格子の周期）ｄ０の２倍を示す。本発明においては、位相シフト量を予めΛ／４より小さくしておくことで、主モードをストップバンドの中央で発振させる。位相シフト量が（９／４０）Λより大きいと、通常駆動時の電流を注入した際に、主モードがストップバンド中央より長波長側に移動すると共に、短波長側のピークが成長し単一モード発振が得られなくなる。一方、位相シフト量が（８／４０）Λより小さいと、通常駆動時の電流を注入した際に、主モードがストップバンド中央より短波長側になる。 The DFB-LD according to the present invention will be described. A schematic plan view showing the structure of the DFB-LD is shown in FIG. The DFB-LD 1 has a diffraction grating 6 below the multi-quantum well (MQW) active layer 4 and non-reflective films (AR coating, antireflection films) 7a and 7b on both end faces. Have The diffraction grating 6 has a phase shift unit 6a. The phase shift amount d of the phase shift unit 6a in the present invention is (8/40) Λ to (9/40) Λ. Here, Λ indicates twice the interval (diffraction grating period) d 0 between the peaks (or valleys) of the diffraction grating 6. In the present invention, the main mode is oscillated at the center of the stop band by making the phase shift amount smaller than Λ / 4 in advance. If the phase shift amount is larger than (9/40) Λ, the main mode moves to the longer wavelength side from the center of the stop band and the short wavelength peak grows and becomes single when the current during normal driving is injected. Mode oscillation cannot be obtained. On the other hand, if the amount of phase shift is smaller than (8/40) Λ, the main mode is on the shorter wavelength side than the center of the stop band when a current during normal driving is injected.

図２に示すＤＦＢ−ＬＤ１においては、位相シフト部６ａは、軸方向の中央に形成してあるが、本発明の有効性は位相シフト部６ａの位置によらない。例えば、出力前後比を変更するために、位相シフト部６ａを素子の中央からずらして形成することができる。このとき、位相シフト部６ａと素子の中央との距離が大きくなるほど、ストップバンド端ピークが成長しやすくなるが、本発明によれば、位相シフト量をΛ／４以下にすることにより、ストップバンド端ピークの成長を抑制することができる。   In the DFB-LD1 shown in FIG. 2, the phase shift unit 6a is formed at the center in the axial direction, but the effectiveness of the present invention does not depend on the position of the phase shift unit 6a. For example, in order to change the output front-rear ratio, the phase shift portion 6a can be formed shifted from the center of the element. At this time, the longer the distance between the phase shift unit 6a and the center of the element, the easier the stop band end peak grows. However, according to the present invention, the stop band can be reduced by setting the phase shift amount to Λ / 4 or less. Edge peak growth can be suppressed.

回折格子６及び位相シフト部６ａは、電子ビーム（ＥＢ；Electron Beam）描画法を使用することにより、精密に形成することができる。   The diffraction grating 6 and the phase shift part 6a can be precisely formed by using an electron beam (EB) drawing method.

本発明による効果を確認するために、位相シフト量及び結合係数が異なるＤＦＢ−ＬＤを作製し、それぞれのＤＦＢ−ＬＤについて閾値電流レベル時と通常駆動レベル時のスペクトルを観察した。作製したＤＦＢ−ＬＤは、図２に示すような構造であって、両端面に無反射膜を有するＭＱＷ活性層の１．５５μｍ帯ＤＦＢ−ＬＤである。素子長Ｌは、４５０μｍであり、位相シフト部は、前から１９５μｍ（後ろから２５５μｍ）の位置に形成した。位相シフト部を素子中央から若干前方側にずらしてあるのは、出力前後比を前方側に上げるためである。位相シフト量は、グレーティング深さの指標である結合係数κＬ＝２．５及び３．５のそれぞれの回折格子において、電子ビーム描画時のピッチで（１１／４０）Λ、（１０／４０）Λ（＝Λ／４）、（９／４０）Λ、及び（８／４０）Λに設定した。   In order to confirm the effect of the present invention, DFB-LDs having different phase shift amounts and coupling coefficients were produced, and the spectrums at the threshold current level and the normal drive level were observed for each DFB-LD. The manufactured DFB-LD is a 1.55 μm band DFB-LD having an MQW active layer having a structure as shown in FIG. The element length L was 450 μm, and the phase shift portion was formed at a position of 195 μm from the front (255 μm from the back). The reason why the phase shift unit is shifted slightly forward from the center of the element is to increase the output front-rear ratio to the front side. The phase shift amounts are (11/40) Λ and (10/40) Λ at the pitch at the time of electron beam writing in each diffraction grating having a coupling coefficient κL = 2.5 and 3.5, which is an index of the grating depth. (= Λ / 4), (9/40) Λ, and (8/40) Λ.

各ＤＦＢ−ＬＤについて、閾値電流レベルの電流として、閾値電流の０．９倍の電流（０．９Ｉｔｈ）（約１０ｍＡ）、及び駆動レベルの電流としてレーザ発振している３０ｍＡ（光電力Ｐｏ＝約５ｍＷ）を流した。このとき観測したスペクトルを図３〜図６に示す。図３は、結合係数κＬ＝２．５のＤＦＢ−ＬＤに、０．９Ｉｔｈの電流を流したときのスペクトルであり、図４は、結合係数κＬ＝２．５のＤＦＢ−ＬＤに、３０ｍＡの電流を流したときのスペクトルである。また、図５は、結合係数κＬ＝３．５のＤＦＢ−ＬＤに、０．９Ｉｔｈの電流を流したときのスペクトルであり、図６は、結合係数κＬ＝３．５のＤＦＢ−ＬＤに、３０ｍＡの電流を流したときのスペクトルである。各図において、（ａ）は、電子ビーム描画時の位相シフト量の設定が（１１／４０）Λ、（ｂ）は（１０／４０）Λ、（ｃ）は（９／４０）Λ、そして（ｄ）は（８／４０）Λである。 For each DFB-LD, the current at the threshold current level is 0.9 times the current (0.9I th ) (about 10 mA), and the current at the drive level is 30 mA (optical power Po = About 5 mW). The spectrum observed at this time is shown in FIGS. 3, the DFB-LD of the coupling coefficient .kappa.L = 2.5, a spectrum at a current of 0.9I th, 4, the DFB-LD of the coupling coefficient .kappa.L = 2.5, 30 mA It is a spectrum when the current of is passed. Further, FIG. 5, the DFB-LD of the coupling coefficient .kappa.L = 3.5, a spectrum at a current of 0.9I th, 6, the DFB-LD of the coupling coefficient .kappa.L = 3.5 , The spectrum when a current of 30 mA is passed. In each figure, (a) is the phase shift amount setting (11/40) Λ, (b) is (10/40) Λ, (c) is (9/40) Λ, and (D) is (8/40) Λ.

結合係数が２．５及び３．５のどちらのＤＦＢ−ＬＤにおいても、電流値が閾値電流付近である場合には、位相シフト量が（１０／４０）Λの時に（図３（ｂ）及び図５（ｂ））、主モードがストップバンドの中央に位置している。位相シフト量が（１１／４０）Λの時は（図３（ａ）及び図５（ａ））、主モードは中央より若干長波長側に位置し、（９／４０）Λ、及び（８／４０）Λの時は（図３（ｃ）、（ｄ）及び図５（ｃ）、（ｄ））、主モードは中央より若干短波長側に位置している。ところが、電流値を３０ｍＡにすると、いずれのＤＦＢ−ＬＤにおいても主モードは長波長側にずれている。例えば、図４（ｂ）及び図６（ｂ）に示す、位相シフト量が（１０／４０）ΛのＤＦＢ−ＬＤにおいては、主モードがストップバンド中央より長波長側に移動し、これに伴い、ストップバンド短波長側のピークが増大している。特に、κＬ＝３．５のＤＦＢ−ＬＤにおいてこの特徴は顕著になっている。実際に、両端面に無反射膜を有するＤＦＢ−ＬＤにおいては、ストップバンド短波長側ピークの成長が、単一モード発振が得られない主要因となっている。一方、図４（ｃ）、（ｄ）及び図６（ｃ）、（ｄ）に示す、位相シフト量が（９／４０）Λ及び（８／４０）ΛのＤＦＢ−ＬＤにおいては、電流値３０ｍＡ時で主モードがストップバンドの中央となっており、このとき、ストップバンド短波長側ピークの成長が抑制されている。したがって、電子ビーム描画時の設定において、位相シフト量を（１０／４０）Λより大きくすると、駆動レベルの電流注入時に単一モード発振を崩しやすいが、位相シフト量を（８／４０）Λ〜（９／４０）Λにすると、駆動レベルの電流注入時の安定な単一モード発振に有効であることができることが確認された。   In both DFB-LDs with coupling coefficients of 2.5 and 3.5, when the current value is in the vicinity of the threshold current, the phase shift amount is (10/40) Λ (see FIG. 3B and FIG. 5 (b)), the main mode is located at the center of the stop band. When the phase shift amount is (11/40) Λ (FIGS. 3A and 5A), the main mode is located slightly longer than the center, and (9/40) Λ and (8 In the case of / 40) Λ (FIGS. 3C and 3D and FIGS. 5C and 5D), the main mode is located slightly shorter than the center. However, when the current value is set to 30 mA, the main mode is shifted to the long wavelength side in any DFB-LD. For example, in the DFB-LD whose phase shift amount is (10/40) Λ shown in FIG. 4B and FIG. 6B, the main mode moves to the longer wavelength side from the center of the stop band. The peak on the short wavelength side of the stop band is increasing. In particular, this feature is remarkable in DFB-LD with κL = 3.5. Actually, in the DFB-LD having antireflection films on both end faces, the growth of the stopband short wavelength side peak is the main factor that single mode oscillation cannot be obtained. On the other hand, in the DFB-LDs having phase shift amounts of (9/40) Λ and (8/40) Λ shown in FIGS. 4C, 4D, 6C, and 6D, the current value At 30 mA, the main mode is the center of the stop band, and at this time, the growth of the stop band short wavelength side peak is suppressed. Therefore, if the phase shift amount is set larger than (10/40) Λ in the setting at the time of electron beam drawing, the single mode oscillation is likely to be destroyed at the time of driving level current injection, but the phase shift amount is set to (8/40) Λ˜ It was confirmed that (9/40) Λ can be effective for stable single mode oscillation at the time of drive level current injection.

実施例１において作製したκＬ＝２．５のＤＦＢ−ＬＤにおける発振時の光Ｆｉｅｌｄ分布を図７に示す。発振時の光Ｆｉｅｌｄ分布は、位相シフト部の位置で極大となる。この光Ｆｉｅｌｄの大きいところでは誘導放出によりキャリア消費が大きくなる。これにより、位相シフト部のキャリア密度は他の部分に対して相対的に低くなり、導波路屈折率は増大する。この現象は、光出力が高いとき、すなわち駆動電流が大きいほど顕著となる。したがって、電流注入の増大に伴い、位相シフト部における光学路長が増大する。そこで、この光学路長の増大分を考慮し、回折格子形成時に位相シフト量をΛ／４より予め若干小さくすることで、好ましくは位相シフト量を（８／４０）Λ〜（９／４０）Λにすることで、単一モードの発振を実現することが可能になると考察される。   FIG. 7 shows an optical field distribution during oscillation in the DFB-LD with κL = 2.5 manufactured in Example 1. FIG. The light field distribution during oscillation becomes maximum at the position of the phase shift unit. Where the optical field is large, carrier consumption increases due to stimulated emission. Thereby, the carrier density of the phase shift portion is relatively low with respect to other portions, and the waveguide refractive index is increased. This phenomenon becomes more prominent when the light output is high, that is, as the drive current is larger. Therefore, as the current injection increases, the optical path length in the phase shift portion increases. In view of this increase in the optical path length, the phase shift amount is preferably made slightly smaller than Λ / 4 at the time of forming the diffraction grating, so that the phase shift amount is preferably (8/40) Λ to (9/40). It is considered that a single mode oscillation can be realized by setting Λ.

実施例１においては位相シフト量が（８／４０）Λ〜（１１／４０）ΛのＤＦＢ−ＬＤについてスペクトルを観察したが、実施例２においては、位相シフト量を（８／４０）よりさらに小さくしたときのスペクトルをシミュレーションした。実施例１と同様のＤＦＢ−ＬＤであって、結合係数κＬ＝３．５、位相シフト量（６／４０）ΛのＤＦＢ−ＬＤについてスペクトルシミュレーションを行った。図８にシミュレーション結果を示す。注入電流＝０．９Ｉｔｈ時においては（図８（ａ））、主モードはストップバンド中央より短波長側にあり、注入電流＝０．９５Ｉｔｈ時においては（図８（ｂ））、長波長側のピークが成長し、主モードとサブモードの差がなくなっている。この状態では、単一モード発振は得られず、２モード発振に至ることになる。したがって、位相シフト量を（８／４０）Λよりさらに小さくすると、単一モード発振が得にくいことが確認された。 In Example 1, the spectrum was observed for DFB-LDs having a phase shift amount of (8/40) Λ to (11/40) Λ. In Example 2, the phase shift amount was further increased from (8/40). The spectrum when it was made small was simulated. A spectrum simulation was performed for a DFB-LD similar to that of Example 1 with a coupling coefficient κL = 3.5 and a phase shift amount (6/40) Λ. FIG. 8 shows the simulation result. During the injection current = 0.9I th is (FIG. 8 (a)), the main mode is from stop band center to the short wavelength side, at the time of injection current = 0.95I th (FIG. 8 (b)), the length The peak on the wavelength side grows, and the difference between the main mode and the submode disappears. In this state, single-mode oscillation cannot be obtained, resulting in two-mode oscillation. Therefore, it was confirmed that single mode oscillation is difficult to obtain when the phase shift amount is made smaller than (8/40) Λ.

本発明のＤＦＢ−ＬＤは、上記実施形態及び実施例に限定されることなく、本発明の範囲内において、種々の変形、変更及び改良が可能であることは言うまでもない。例えば、本発明のＤＦＢ−ＬＤは、素子長、波長帯、結合係数κＬ、ＤＦＢ−ＬＤの構造（活性層、クラッド層、光ガイド層、キャップ層、電極等の構成）等の要素に依存することなく適用することができる。さらに、本発明のＤＦＢ−ＬＤは、活性層の上側に回折格子を有する構造にも適用することが可能である。   It goes without saying that the DFB-LD of the present invention is not limited to the above-described embodiments and examples, and various modifications, changes and improvements can be made within the scope of the present invention. For example, the DFB-LD of the present invention depends on factors such as element length, wavelength band, coupling coefficient κL, and structure of the DFB-LD (configuration of active layer, cladding layer, light guide layer, cap layer, electrode, etc.). Can be applied without. Furthermore, the DFB-LD of the present invention can be applied to a structure having a diffraction grating on the upper side of the active layer.

本発明のＤＦＢ−ＬＤにおける動作を説明するためのスペクトル図。The spectrum figure for demonstrating the operation | movement in DFB-LD of this invention. ＤＦＢ−ＬＤの構造を示す概略平面図。The schematic plan view which shows the structure of DFB-LD. 実施例１における結合係数κＬ＝２．５、注入電流＝０．９Ｉｔｈ時のスペクトル図。Spectrum of the coupling coefficient .kappa.L = 2.5, injected current = 0.9I th time in the first embodiment. 実施例１における結合係数κＬ＝２．５、注入電流＝３０ｍＡ時のスペクトル図。FIG. 6 is a spectrum diagram when the coupling coefficient κL = 2.5 and the injection current = 30 mA in Example 1. 実施例１における結合係数κＬ＝３．５、注入電流＝０．９Ｉｔｈ時のスペクトル図。Spectrum of the coupling coefficient .kappa.L = 3.5, injected current = 0.9I th time in the first embodiment. 実施例１における結合係数κＬ＝３．５、注入電流＝３０ｍＡ時のスペクトル図。The spectrum figure in case of the coupling coefficient (kappa) L = 3.5 in Example 1, and injection current = 30mA. 実施例１におけるＤＦＢ−ＬＤ発振時の光Ｆｉｅｌｄ分布図。FIG. 3 is an optical field distribution diagram during DFB-LD oscillation in the first embodiment. 実施例２におけるシミュレーション結果。The simulation result in Example 2. FIG. 位相シフト量Λ／４のＤＦＢ−ＬＤにおけるスペクトル図。The spectrum figure in DFB-LD of phase shift amount Λ / 4. 位相シフト量Λ／４のＤＦＢ−ＬＤにおいて主モード単一発振性が崩れたスペクトル例。An example of a spectrum in which the main mode single oscillation is broken in a DFB-LD having a phase shift amount Λ / 4.

符号の説明Explanation of symbols

１ 分布帰還型半導体レーザ
２ａ、２ｂ 電極
３ａ、３ｂ クラッド層
４ 活性層
５ 光ガイド層
６ 回折格子
６ａ 位相シフト部
７ａ、７ｂ 無反射膜

DESCRIPTION OF SYMBOLS 1 Distributed feedback type semiconductor laser 2a, 2b Electrode 3a, 3b Clad layer 4 Active layer 5 Optical guide layer 6 Diffraction grating 6a Phase shift part 7a, 7b Antireflection film

Claims (1)

位相シフト部を有する回折格子と、
両端面に無反射膜を有する多重量子井戸型活性層と、を備える１．５５μｍ帯の分布帰還型半導体レーザであって、
素子長は４５０μｍであり、
結合係数は２．５又は３．５であり、
前記位相シフト部は端部から１９５μｍの位置に形成され、
前記位相シフト部の位相シフト量は、（８／４０）Λ〜（９／４０）Λ（Λは回折格子間隔の２倍）であることを特徴とする分布帰還型半導体レーザ。 A diffraction grating having a phase shift portion;
A 1.55 μm band distributed feedback semiconductor laser comprising: a multi-quantum well active layer having antireflection films on both end faces ;
The element length is 450 μm,
The coupling coefficient is 2.5 or 3.5,
The phase shift part is formed at a position of 195 μm from the end part,
The phase shift amount of the phase shift unit is (8/40) Λ to (9/40) Λ (Λ is twice the diffraction grating interval).
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